Microstructurally refined multiphase castings

ABSTRACT

The present invention relates to eutectic alloy systems, such as white irons, in which a primary phase grows out of the melt when the melt is cooled below the liquidus temperature and comprises pouring the molten metal alloy at a temperature at or above the liquidus in a stream into a casting mould to form a casting and introducing a particulate material into the stream of molten metal to extract heat from the molten metal alloy to undercool the molten metal alloy from the pour temperature into the primary phase solidification range between the liquidus and the solidus temperatures of the alloy and thereby initiate primary phase nucleation and restrict primary phase growth. The primary function of the particulate material is to act as a heat sink but the particulate material may at least partially dissolve in the melt and may act as a seeding agent for the primary phase. The invention is described with particular reference to high chromium hypereutectic white irons.

This is a continuation of International Application No. PCT/AU94/00264,with an international filing date of May 20, 1994, now abandoned.

This is a continuation of International Application No. PCT/AU94/00264,with an international filing date of May 20, 1994, now abandoned.

TECHNICAL FIELD

The present invention relates to multiphase castings, and isparticularly concerned with a casting method by which it is possible torefine a primary phase which forms out of a melt in a two phase regionof a eutectic system. The invention is applicable to all metal systemswhose solidification characteristics and final microstructures can bedescribed by a eutectic phase diagram. Examples of such systems arealuminium/silicon, lead/tin, lead/antimony, copper/silver and ironalloys, especially white irons.

BACKGROUND OF THE INVENTION

In eutectic systems, solidification of alloys with hypereutectic andhypoeutectic compositions occurs over the temperature range defined bythe liquidus and solidus temperatures for each alloy composition.

During solidification a primary phase forms by a nucleation and growthprocess. The size and distribution of the primary phase is determined,inter alia, by the cooling rate in the temperature interval between theliquidus and solidus. In general, the faster the cooling rate the finerthe grain size and distribution of the primary solid phase.

There are several procedures described in the literature to increase thecooling rate through the solidification range:

(a) Use of minimum liquid metal pouring temperature i.e. just above theliquidus temperature.

(b) Using casting moulds with a higher chill factor than the usualsilica sand based moulds, e.g. zircon sand, chromite sand and variousmetal moulds.

(c) Reducing casting metal thickness.

(d) Use of internal metal chills in the casting.

(e) Using alloys with chemical compositions close to the eutecticcomposition.

These procedures have certain limitations and are not applicable toevery casting material or do not go far enough in the grain refinementprocess to substantially enhance desired material properties.

Some of these procedures, and some limitations, are discussed at lengthin Australian Patent Application AU-A-28865/84 in relation to white castirons, both with hypoeutectic and hypereutectic compositions.AU-A-28865/84 sought to alleviate problems which had been identified inproducing relatively thick section castings of high chromiumhypereutectic white iron, by paying closer attention to themanufacturing variables in order to decrease the primary carbide sizeand to make the microstructure substantially constant throughout thecasting section.

The wear resistant properties of white irons, including high chromiumhypereutectic white irons, have been known for many years, and thelatter alloys are used in the formation of wear resistant parts forlining pumps, pipes, nozzles, mixers and similar devices which are usedto convey fluids containing abrasive particles, for example in mineralprocessing plants. The hypereutectic material consists of acicular M₇ C₃(wherein M=Cr,Fe,Mo,Mn) primary carbides in a matrix, and, in a paper byK. Dolman : Alloy Development : Shredder Hammer Tips, Proceeding ofAustralian Society of Sugar Cane Technology, April 1983, pp 81-87, itwas outlined how the wear resistant properties of these materialsincrease directly with the volume fraction of primary carbide that ispresent in sugar mill hammer tip castings 25 mm thick. However acorresponding decrease in fracture toughness was also noted and in orderto give the hammer tips sufficient toughness they were bonded to mildsteel backing plates. The difficulty in producing thick section castingsbecause of the tendency to crack was also noted.

AU-U-28865/84 aimed to overcome the disadvantages of low fracturetoughness and cracking by providing, in a high chromium hypereutecticwhite iron casting having a volume fraction of primary carbides inexcess of 20% substantially throughout the alloy, a primary carbide meancross-sectional dimension not greater than 75 μm.

Apart from controlling the degree of superheat on pouring of the melt,it was proposed to achieve this aim by cooling the metal at a sufficientrate to restrict the growth of primary carbides. As an example of thisprocedure, a 25 mm thick hammer tip wear component cast in a zirconbearing shell mould was able to achieve a mean primary carbide diameterof 40 μm, with a super chilled zone about 0.5 mm thick formed at theinterface of the mould and casting. However, in order to providesufficient fracture toughness to avoid failure under extreme impactloading the casting had to be brazed to a mild steel backing plate, muchas described in the aforementioned Dolman paper. Larger components, forexample of 35 mm thickness, with sufficient fracture toughness were alsocast with a mean carbide diameter of 40 μm, but only with the assistanceof a permanent mild steel rod insert in the casting. It was specificallynoted that identical castings without the insert had a mean carbidediameter typically about 100 μm and failed the fracture toughness tests.Thus, for alloy castings having a minimum thickness dimension of 30 mm,it was suggested that the insert preferably comprises at least about 10%by weight of the casting. For larger castings, for example having aminimum thickness dimension up to 70 mm, it was suggested that a chillmould be used as well as the insert.

AU-A-28865/84 also proposed the addition of carbide forming elementsmolybdenum, boron, titanium, tungsten, vanadium, tantalum and niobium toincrease the volume fraction of primary carbides due to their strongcarbide forming action. These elements are absorbed within the M₇ C₃carbides of the high chromium hypereutectic melt, to the limit of theirsolubility. Beyond the limit of their solubility, they form secondary orprecipitated carbides within the matrix to provide some microhardeningof the matrix and some increase in erosive wear resistance. It was alsonoted that where the carbide forming elements are present in themetallic form in an amount exceeding about 1.0 wt. %, they providednucleating sites for the M₇ C₃ primary carbides to an extent resultingin grain refinement of the M₇ C₃ carbides.

There is no explanation in AU-A-28865/84 of when or how the metalliccarbide forming elements were included in the melt, but it was suggestedthat the resultant carbides may at least in part come out of solutionand that care was therefore required to ensure they were substantiallyuniformly dispersed in the melt at the time of pouring. It was alsosuggested in relation to the inclusion of metallic carbide formingelements to be desirable that the period for which the melt was heldprior to pouring be kept to a minimum so as to avoid excessive growth ofthe carbide particles.

Instead of including the carbide forming elements in metallic form, theymay according to AU-A-28865/84 be added as their carbides in fineparticulate form. However, it was suggested that the fine particulatecarbides may at least partially remain in suspension rather than gofully into solution in the melt and that this was particularly likelywhere the degree of superheating of the melt was limited. Again,therefore, care was required to ensure that the particulate carbideswere substantially uniformly dispersed in the melt at the time ofpouring the melt.

The addition of particulate material to the melt in order to increasethe volume fraction of primary carbides as proposed in AU-A-28865/84 hasnot been practised in the art of hypereutectic white irons before thepresent invention.

U.S. Pat. No. 3,282,683 proposed the manufacture of an improved whiteiron having smaller, so-called undercooled or plate-type, carbides andincreased toughness by the addition to the melt in the ladle, prior topouring, of a carbide stabilizing or metastabilizing agent selected froma large number of elements. Similar undercooling by the addition ofcarbide metastabilizing agents to a nodular cast iron melt in the ladleis proposed in U.S. Pat. No. 2,821,473.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of refiningthe primary phase in cast eutectic alloy systems by the addition ofparticulate material to the melt in which control of the primary phasegrowth is improved compared to the prior art described above.

According to the present invention there is provided a method of castinga metal alloy of a eutectic alloy system comprising.

(a) forming a melt of the metal alloy;

(b) pouring the molten metal alloy at a temperature at or above theliquidus temperature in a stream into a casting mould to form a casting,and

(c) introducing a particulate material into the stream of molten metalto extract heat from the molten metal alloy to undercool the moltenmetal alloy from the pour temperature into the primary phasesolidification range between the liquidus and the solidus temperaturesof the metal alloy.

Further according to the present invention there is provided an alloycasting when formed by the method described in the immediately precedingparagraph.

By substantially instantaneously extracting heat from the melt andundercooling the melt only as it is poured, the particulate materialoptimises the conditions for promoting the formation of a fine grainstructure by maximising primary phase nucleation after the pour hasstarted and thereby minimising primary phase growth, without the needfor special moulds, chill plates and/or metal inserts. In addition, noseparate stirring of the melt is required to ensure that the particulatematerial is thoroughly dispersed since the particulate material can beadequately dispersed as it is introduced to the melt during the pour orby movement of the melt in the mould as it is poured. In contrast, forexample, to the proposal in AU-A-28865/84 of adding fine particulatecarbides of carbide forming elements, the present invention reduces thetime during which primary phase growth can occur, thereby bettercontrolling the grain refinement, and optimises the uniform dispersal ofthe particulate material and therefore of primary phase nucleationwithout the need for separate stirring equipment for the melt in theladle, thereby better controlling the uniform distribution of theprimary phase. The particulate material may also act as a seed toprovide primary phase nucleation and increased primary phase volumes,but the primary phase volume proportion is better able to be increasedby virtue of the grain refinement allowing more primary phaseconstituent (e.g. carbon for carbide primary phase) to be included inthe initial melt while avoiding the problems of the prior art, such ascracking.

A further advantage of the present invention is that it may allow alarger pouring window for the casting, which is highly beneficial inpractice. Without the addition of the particulate material a melt mustgenerally be poured within a narrow temperature window to ensure thedesired physical properties are achieved, for example no more than 15°C. above liquidus, which is very difficult to achieve under foundryconditions. The increased rate of cooling provided during the pour bythe addition of the particulate material accordance with the inventionallows the pouring window to be increased, for example upto 30° C. ormore above liquidus in the case of the previous 15° C. window, whilemaintaining or even reducing the final size of the primary phase.

The particulate material is preferably added to the melt uniformlythrough the pour, but the addition may be varied, interrupted or delayedif, for example, the same degree of grain refinement is not requiredthroughout the casting

The particulate material may be introduced to the final pour of the meltin any suitable manner, but preferably by injection through a nozzle.Injection may be performed in a carrier gas of, for example, compressedair or inert gas. Suitable injection equipment is the Wedron FF40 powderinjection system or powder injection equipment manufactured by Foseco.The pour may be performed in the usual manner, for example by top orbottom casting from a ladle or from a tundish.

The amount of fine particulate material added to the melt may bedependent upon a variety of conditions, for example the degree ofsuperheat, the level of undercooling required, the desired volumefraction of primary phase, the size of the casting and the degree ofgrain refinement. The preferred rate is in the range of 0.1 to 10% ofthe final casting weight, below which the effect may be minimal andabove which the grain refinement may not be able to be controlledsatisfactorily. A more preferred range is 0.1 to 5% of the final castingweight, and most preferably the addition is in a range from about 0.5%to about 1% of the final casting weight.

Advantageously, any type of element or compound that is not detrimentalto the casting may be used as the particulate material, since theprimary requirement is that the particulate material extracts heat fromthe melt and by that undercooling initiates multiple primary phasenuclei. Suitable types of material will vary with the melt. Preferablythe particulate material is a metal or inorganic metal compound.Advantageously, the material is capable of at least partially meltingand/or dissolving in the melt, but the material may be absorbed, atleast in part, within the primary phase. One type of material that issuitable is a metal that is an integral part of the usual meltcomposition, for example particulate lead in a 30% tin/lead system (at apour temperature of about 265° C.), particulate antimony in a 50%antimony/lead system (at a pour temperature of about 490° C.),particulate copper in a 30% silver/copper system (at a pour temperatureof about 940° C.) and particulate iron, white iron (eg 27% Cr) or steelin an iron alloy such as a white iron. Other metal or metal compoundswhich may be suitable arm those which have a strong primary phaseseeding action including, for high chromium hypereutectic white ironcastings, those mentioned in AU-A-28865/84, namely one or more ofmolybdenum, boron, titanium, tungsten, vanadium, tantalum and niobium,whether as the metal or in carbide form. Still other materials which maybe most suitable are those having a compatible crystallographicstructure with the primary phase, for example, in the case of the M₇ C₃primary carbides of a high chromium hypereutectic white iron, highcarbon ferrochrome and chromium carbide, since they can act as seedingsites for the primary phase in addition to providing rapid undercooling.

The particulate material, which is conveniently in powder form,preferably has a maximum particle size of no more than 200 μm, morepreferably no more than 150 μm, since particles that arc too large mayprovide the required thermal mass effect but be ineffective in providingthe desired grain refinement. Particles that are too small, for examplewith a maximum particle size of less than 5 to 10 μm, may be effectiveas a heat sink but may not be effective as seeding agents if they fullydissolve in the melt. More preferably the mean particle size of theparticles is in the range 20 to 100 μm and the maximum particle size isno more than 75 μm. It may be advantageous for the maximum particle sizeto be no more than 50 μm.

Although the invention is applicable generally to multiphase castings,it is especially applicable to eutectic systems in which the primaryphase can grow as a coarse, discrete phase. An example of such a systemis high chromium hypereutectic white iron and, for convenience only, theinvention will be further described with specific reference to thisalloy.

The principal objective of the research that led to the presentinvention was to refine the microstructure of thick sectionhypereutectic white iron castings significantly more than was possibleusing conventional prior art casting technology. Hypereutectic whiteirons have offered the potential for significant wear improvementbecause of the high volumes of the very hard M₇ C₃ primary carbideswhich could be formed. However, at these very high carbide levels thecasting microstructure could not be produced at a fine enough size togive sufficient physical properties for a practical casting. Inaddition, the maximum carbon level in the prior art has been dictated bythe maximum size of primary carbide which is subsequently formed andwhich determines the soundness of the final casting. By refining themicrostructure a much higher carbon content and therefore volume ofprimary carbide can be utilised within the hypereutectic white iron,thereby enabling an increase not only in fracture toughness but also inwear resistance.

High chromium hypereutectic white iron comprises from about 3 to about85 wt % carbon, from about 20 to about 45 wt % chromium and optionalalloying additions of one or more of copper, manganese, molybdenumsilicon and nickel as well as boron and other carbide forming elements,balance predominantly iron and incidental impurities including elementsdeed from the particulate material. The alloying additives in the moltenmetal composition preferably include, by weight up to about 15%manganese, up to about 10% molybdenum, up to about 10% nickel, up toabout 3% silicon, up to about 5% copper and up to about 2% boron as wellas up to about 10% derived from the particulate material. Up to about 1wt % each of phosphorous and sulphur may also be included. Preferredcompositions consist essentially of 4 to 5.5 wt % C, 28 to 37 wt % Cr, 1to 4 wt % Mn, 0.1 to 1 wt % Si, 0.5 to 1.5 wt % Mo, less than 1 wt % Ni,less than 0.1 wt % P, less than 0.1 wt % S, balance Fe and incidentalimpurities.

It has been found that by the use of the present invention in castinghigh chromium hypereutectic white irons the M₇ C₃ primary carbides canbe substantially uniformly distributed throughout the casting with amean cross-sectional dimension in a range of about 10 to 50 μm,preferably 15 to 45 μm, most preferably 20 to 30 μm. However, the meancross-sectional dimension of the M₇ C₃ primary carbides (hereinaftersometimes referred to as the "carbide size") is dependent among otherthings on the degree of superheat and the size of the casting, andacceptable castings may be produced with M₇ C₃ primary carbide meancross-sectional sizes above these ranges but with more freedom beingpermitted by the invention in the degree of superheat during castingand/or in the size of the casting. In particular, high chromiumhypereutectic white iron castings with cross-sectional dimensions of 50to 100 mm or more can readily be made by the invention with acceptablephysical properties without the use of internal chills or the like.

In general, the optimum pouring temperature at which the particulatematerial is added to a melt is dependent on the liquidus temperature,casting section size, and the amount of powder added, and the preferredpouring temperature (°C.) for a high chromium hypereutectic white ironmelt may be defined by the formula:

    ______________________________________                                        liquidus (°C.) + A + 15B                                               ______________________________________                                        where A                                                                              =     15° C. for casting section thickness less than 50 mm             =     10° C. for casting section thickness from 50 to 100                    mm                                                                      =     5° C. for casting section thickness greater than 100                   mm.                                                              .B     =     amount of particulate material in weight %.                      ______________________________________                                    

The same formula may be applicable to other melts, but in relation tothe high chromium hypereutectic white iron melt the formula is aimedprimarily at achieving a carbide size of 25 μm.

The M₇ C₃ primary carbides in the high chromium hypereutectic white ironwill normally exist in a matrix of eutectic carbide and martensite withretained austenite. The M₇ C₃ primary carbides will generally beacicular and with much the same aspect ratio as in the prior art whiteirons. Because of the relatively small M₇ C₃ primary carbides achievableby the method of the invention, it is now practical to subject the highchromium hypereutectic white iron castings to hardening by heattreatment without cracking the castings. Secondary carbides may developas a result of heat treatment or from the melt. The heat treatment maybe an age hardening procedure such as by soaking at from 750° to 1050°C. for, for example 2 to 5 hours at 900° to 1000° C., followed by air orfurnace cooling. Alternatively, the casting may be subjected to a heattreatment such as cryogenic chilling, for example down to minus 200° C.

The minimum M₇ C₃ primary carbide content in the high chromiumhypereutectic white iron is preferably of the order of 20 volume %, buta far higher M₇ C₃ primary carbide content, for example up to 50 volume% or higher is possible. Such levels of M₇ C₃ primary carbide contentwould lead to very brittle castings and possibly cracking without thegrain refinement also achievable by the present invention. The eutecticphase is generally accepted as containing of the order of 30% eutecticM₇ C₃ carbides.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of method in accordance with the invention will nowbe described by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 is an optical photomicrograph at 100x magnification of the ladleinoculated high chromium hypereutectic white iron casting of Example 1;

FIG. 2 is an optical photomicrograph at 100x magnification of the mouldinoculated high chromium hypereutectic white iron casting of Example 1,having the same melt composition as the casting of FIG. 1;

FIG. 3 is a graph showing a Vicker's Hardness traverse through the fullthickness of the mould inoculated casting of Example 1;

FIG. 4 is an optical photomicrograph at 100x magnification of the highcarbon mould inoculated casting of Example 2;

FIG. 5 is a scanning electron microscope back scattered image of thecasting of Example 3 which has been mould inoculated at a superheat of30° C.;

FIG. 6 is a graph showing the relationship between the degree ofsuperheat, the amount of mould inoculation and the primary carbide sizeas described in Example 5;

FIG. 7 is a graph showing the relationship between primary carbide andcasting hardness as described in Example 5;

FIG. 8 is a graph showing the relationship between wear rate and primarycarbide size, both as cast as described in Example 5 and after heattreatment as described in Example 6; and

FIG. 9 is a graph comparing hardness before and after heat treatment asdescribed in Example 7.

EXAMPLES

The following examples are given to further illustrate the invention inrelation to various compositions of high chromium hypereutectic whiteiron. They have been selected for convenience only and are not intendedto limit the invention in any way. In all of the examples in accordancewith the invention powder material was injected into a stream of a highchromium hypereutectic white iron melt, as it was poured into the mould,with compressed air using a Wedron FF40 powder injection system runningat a feed rate of 9 kg/min. This is sometime referred to as "mouldinoculation" in the Examples.

Example 1

A chromium carbide powder having a particle size range of minus 150 μmwas injected into the liquid metal at a delivery rate of 10 kg of powderper tonne of liquid metal (1%) in two different ways: a) by addition tothe ladle at about 100° C. superheat (ladle inoculation) shortly beforepouring into the casting mould; b) by introduction into the moltenstream during filling of the mould (mould inoculation). The castingswere of an impeller having a maximum thickness of 150 mm. The sectionanalyzed had a thickness of 40 mm.

The compositions, conditions and results of the as cast material are setout in Table 1. The reduction in primary carbide cross-sectionaldimension is dearly evident from the photomicrographs (mag: 100x) ofFIGS. 1 (ladle inoculation) and 2 (mould inoculation).

                  TABLE 1                                                         ______________________________________                                                      Ladle Inoculation                                                                       Mould Inoculation                                     ______________________________________                                        Composition wt. %                                                             Cr              29.97       3008                                              C               4.31        4.039                                             Mn              2.04        2.03                                              Si              0.55        0.56                                              Mo              0.99        0.98                                              Ni              0.30        0.30                                              Fe              bal         bal                                               Pour Temp °C.                                                                          1464        1369                                              Liquidus °C.                                                                           1364        1364                                              Primary Carbide volume %                                                                      25          25                                                Mean Primary Carbide size μm                                                               40          20-25                                             Hardness: Vickers HV 0.05 g                                                                   699         694                                               ______________________________________                                    

The fractured surface of the mould inoculated impeller exhibited anappearance typical of a fine grain structure throughout the 40 mmthickness of the casting and FIG. 3 illustrates the results of a VickersHardness traverse through the full thickness. A surface hardness ofabout 780 HV dropped to about 650 HV at a depth of about 8-10 mm belowthe surface.

The ladle inoculated casting showed a hypereutectic microstructureconsisting of primary M₇ C₃ carbides having a mean cross-sectionaldimension of 40 μm with a matrix of eutectic carbides with martensiteand retained austenite. There was no evidence of undissolved chromiumcarbide in the microstructure.

The mould inoculated casting showed a fine hypereutectic microstructureconsisting of primary M₇ C₃ carbides having a mean cross-sectionaldimension of less than 25 μm (and therefore about half of the ladleinoculated sample) with very fine eutectic carbides in anaustenite/martensite matrix. Some relatively coarse carbide particleswere in evidence, typical of partially dissolved chromium carbide. Themartensite was present as a consistent layer around all primary andeutectic carbides and appears to have initiated at the carbide/ferrousmatrix interface with growth occurring into the austenite phase. Itspresence would tend to enhance wear resistance and lower the toughnessof the material.

The presence of undissolved large chromium carbide particles in thecasting indicated that the particle size of the powder, nominally lessthan 150 μm, was not optimum. The larger particles in the powder areinefficient in seeding the primary carbides in the microstructure. Thepowder also contained a substantial amount of very fine particles thatare nominally less than 10 μm. These particles would fully dissolve inthe melt and would be effective in rapidly reducing the temperature ofthe liquid but would not be effective as seeding agents for carbideformation. A maximum particle size of about 75 μm is consideredappropriate.

In conclusion, the introduction of 1 wt % chromium carbide powder to thestream of melt was sufficient to rapidly undercool the liquid metal froma superheat of about 5° C. to a temperature just below the liquidus andwithin the two phase (liquid+carbide) region due to a thermal masseffect and thereby restrict the growth of the primary M₇ C₃ carbides. Inaddition, the chromium carbide powder, having the same crystal structureand a higher melting point than the primary M₇ C₃ carbides, acted as acompatible and effective seeding agent for nucleating multiple primarycarbides in the casting.

Example 2

This example considered a high chromium hypereutectic white iron castingcontaining 5.5 wt % carbon and mould inoculated with chromium carbidepowder at a rate of 1% of the final casting weight.

An upper carbon limit of 4.5 wt % had previously been imposed on thestandard composition of high chromium hypereutectic white iron becauseprimary M₇ C₃ carbide coarsening was considered excessive above thatlimit. However, higher carbon levels lead to higher carbide contents inthe microstructure and hence greater wear resistance.

The composition, conditions and results of the as cast material are setout in Table 2. The photomicrograph of FIG. 4 (mag: 100x) illustratesthe hypereutectic microstructure exhibiting a high volume fraction ofprimary M₇ C₃ carbides with some irregular CrC carbides being evident.Higher magnifications illustrate the ferrous matrix showing somemartensite and secondary carbide precipitants.

A visual examination of the casting revealed there was some evidence ofcarbide needles with an estimated maximum length of 3 mm. This issomewhat finer than the carbide size observed in standard (4.5 wt % C)high chromium hypereutectic white iron castings. Gas holes due totrapped air were observed near the top surface of the casting. Thesurface gas holes may be eliminated with the use of a higher pouringtemperature of 1425°-1430° C. or a reduction in the carbon content, forexample to 5.0 wt %. Some coarse undissolved chromium carbide particleswere noted in the microstructure, but it is considered these can beeliminated with a smaller inoculation powder size, for example minus 75μm.

In conclusion, mould inoculation with 1 wt % chromium carbide powder ofa high chromium hypereutectic white iron melt which has a carbon contentof 5.5 wt % is effective in maintaining a primary M₇ C₃ carbide meancross-sectional dimension below about 50 μm. The addition of theinoculation powder to the melt compensated for the adverse effects ofthe higher carbon content.

                  TABLE 2                                                         ______________________________________                                        Composition wt. %                                                             Cr                   30.9                                                     C                    5.55                                                     Mn                   1.99                                                     Si                   0.61                                                     Mo                   1.54                                                     Ni                   0.53                                                     Fe                   bal                                                      Inoculant Particle Size μm                                                                      -150                                                     Inoculation/Pour Temp °C.                                                                   1420                                                     Liquidus °C.  1407                                                     Primary Carbide volume %                                                                           62                                                       Mean Primary Carbide size μm                                                                    50                                                       Harndess: Vickers HV 0.05 g                                                                        730                                                      ______________________________________                                    

Example 3

This example describes the effect of increasing the degree of superheatto 30° C. on the mould inoculation with 1 wt % chromium carbide powderof a standard high chromium hypereutectic white iron. It also examinesthe role of the original CrC inoculating particles in the finalmicrostructure of the casting.

The composition, conditions and results of the as cast 30° C. superheatmaterial are set out in Table 3.

                  TABLE 3                                                         ______________________________________                                        Composition wt. %                                                             Cr                   30.6                                                     C                    4.31                                                     Mn                   2.01                                                     Si                   0.70                                                     Mo                   1.5                                                      Ni                   0.56                                                     Fe                   bal                                                      Inoculant Particle Size μm                                                                      -150                                                     Pour Temp °C. 1400                                                     Liquidus °C.  1370                                                     Primary Carbide Vol %                                                                              25                                                       Mean Primary Carbide Size μm                                                                    50                                                       Hardness: Vickers HV 0.05 g                                                                        681                                                      ______________________________________                                    

The mould inoculation of a standard high chromium hypereutectic whiteiron melt with chromium carbide at a rate of 1% of the final castingweight and at a superheat of 30° C. produced a primary M₇ C₃ carbidesize of 50 μm. However, some macroshrinkage and microshrinkage wereobserved and this could be attributed to the pouring temperature beingtoo high or to the amount of inoculation powder added being insufficientto undercool the melt below the liquidus temperature during inoculation.Some partially dissolved CrC carbide particles were observed and somesecondary carbide precipitation was evident in the ferrous matrix.

A secondary electron image of the microstructure of the 30° C. superheatmould inoculated casting is shown in FIG. 5. Dark central cores in thethree relatively coarse carbides were shown by microanalysis to containchromium only and were consistent with the stoichemistry of the Cr₇ C₃carbides. Lighter outer rims of these castings contain iron and chromiumconsistent with the stoichemistry of (Fe, Cr)₇ C₃ carbides. This showsthat the partially dissolved Cr₇ C₃ powder particles have acted as seedsfor the growth (Fe, Cr)₇ C₃ carbides in the microstructure. This isevidence that the addition of CrC powder to the high chromiumhypereutectic white iron melt has a two fold effect on the finalmicrostructure. 1) rapid undercooling of the molten metal to atemperature below the liquidus line; and 2) the partially dissolved Cr₇C₃ particles acting as effective seeds for nucleation and growth of theprimary M₇ C₃ carbides. This occurs because the crystal structures (unitcell type, size and lattice parameters) for the carbides Cr₇ C₃ and (Fe,Cr)₇ C₃ are compatible, and in fact almost identical.

Analysis of the ferrous matrix also shows that its carbide/matrixboundary regions are lighter than portions between the boundary regions.This indicates that the lighter boundary regions are chromium depleted.During formation of the chromium rich primary carbides, chromium isremoved from the immediate surrounding regions causing coring in thefinal ferrous matrix. The observed presence of martensite in theseboundary regions in Examples 1 and 2 is attributed to the presence of achromium depleted zone in the ferrous matrix.

Example 4

This example compares the casting of Example 3 with two castings fromidentical melts but with one casting identically mould inoculated exceptat a superheat of 15° C. and with the other casting not inoculated atall. This was used to show that the thermal mass cooling of the moltenmetal by the inoculation may be a method of expanding the relativelysmall range of pour temperatures which have been applicable in the pastfor the manufacture of high chromium hypereutectic white iron castingswith acceptable carbide sizes.

The mould inoculation of a high chromium hypereutectic white iron meltwith 1 wt % chromium carbide at a superheat of 30° C. produced a primarycarbide size of 50 μm. This is similar to the same melt cast at asuperheat of 15° C. with no inoculation. However, as compared to theshrinkage described in Example 3, the casting at a superheat of 15° C.with no inoculation was sound.

The same mould inoculation as in Example 3 but at a superheat of 15° C.yielded a casting with a mean primary M₇ C₃ carbide cross-sectionaldimension of 25 μm, but gas holes near the surface which suggests thepouring and inoculation temperature was slightly too low.

It can be shown that the addition of each 1.0 wt % powder to the melt bymould inoculation is equivalent to a 15° C. temperature drop in themolten melt. From this it can be shown that the optimum pouringtemperature for the effective mould inoculation of high chromiumhypereutectic white iron castings where the required mean primary M₇ C₃carbide size is 25 μm is dependent on a) liquidus temperature, b)casting section size and c) amount of inoculant added, according to thefollowing empirical formula:

    ______________________________________                                        Pouring Temperature (°C.)                                                            =     Liquidus Temperature (°C.) + A                     ______________________________________                                                            + 15B                                                     where A       =     15° C. for a casting section                                           thickness less than 50 mm                                               =     10° C. for casting section                                             thickness from 50 to 100 mm                                             =     5° C. for casting section                                              thickness greater than 100 mm.                            and B         =     % inoculant powder of the final                                               casting weight.                                           ______________________________________                                    

As a rough rule for white iron castings, it may be said that a castingthickness of 50 mm is equivalent to a final casting weight of 100 kg anda casting thickness of 100 mm is equivalent to a final casting weight of500 kg.

Example 5

This example compares mould inoculation using 1) high carbon ferrochrome(Fe-Cr) powder (-75 μm), 2) CrC powder (1-150 μm) and 3) iron powder(-200 μm) of high chromium hypereutectic white iron melts at injectionrates ranging from 1 to about 2.5% of the final casting weight and atsuperheats varying from 10° to 40° C., to determine the effect of thevariables on microstructure, hardness and wear resistance compared withthe standard high chromium hypereutectic white iron. All trials werecarried out on an impeller weighing 450 kg.

In this and subsequent examples using high carbon ferrochrome ofnominally minus 75 μm particle size, a sizing analysis shows that theapproximately 90% of the powder has a particle size between 10 and 60μm. Chemical analysis shows the following wt % composition: 8.42% C,69.1% Cr, 0.71 Mn, 1.31% Si, 0.06% Mo and 0.27% Ni.

Table 4 sets out the chemical composition of the castings examined.Sample pieces 70×50×40 mm were cast with the impellers for each melt andwere tested by 1) visual examination, 2) metallography, 3) hardnesstesting, 4) wear testing, and 5) chemical analysis. The chemicalanalysis results set out in Table 4 show that all samples were withinspecification. The chemical analysis also showed the presence of sulphurand phosphorous, but each at less than 0.05 wt %, and of boron, but atless than 0.002 wt %.

                  TABLE 4                                                         ______________________________________                                               Composition wt. % - balance Fe                                         Sample   Cr     C        Mn   Si     Mo   Ni                                  ______________________________________                                        A851     31.17  4.33     2.00 0.55   1.05 0.30                                A852     30.78  4.40     2.01 0.49   1.07 0.30                                A853     30.61  4.38     2.05 0.59   1.05 0.30                                A854     30.55  4.42     2.07 0.62   1.07 0.30                                A855     30.82  4.28     2.02 0.55   1.05 0.30                                A856     30.66  4.36     2.04 0.56   1.07 0.30                                A857     35.28  4.92     2.09 0.73   1.02 0.32                                A858     35.31  4.91     2.05 0.64   1.01 0.32                                A859     34.85  4.80     2.02 0.53   1.01 0.18                                A860     30.23  4.36     2.18 0.57   0.99 0.19                                A861     30.23  4.36     2.18 0.57   0.99 0.19                                A862     30.25  4.40     2.15 0.58   0.99 0.19                                A863     30.25  4.40     2.15 0.58   0.99 0.19                                A864     29.97  4.46     2.19 0.54   0.99 0.19                                A865     29.97  4.46     2.19 0.54   0.99 0.19                                A866     30.39  4.35     2.15 0.54   0.98 0.19                                ______________________________________                                    

Visual Examination

Examination of the fracture faces of the samples revealed a very finefracture face (mean primary M₇ C₃ carbide cross-sectional dimension of50 μm or less) on all mould inoculated samples except A859, a relativelyhigh carbon melt inoculated at a relatively high superheat. The twonon-inoculated castings, A851 and A866 showed the normal coarse fractureface.

Examination of the surface finish of the castings showed all castingswere satisfactory and there was no evidence of cold folds or shrinkagein the impeller castings.

Inspection after machining the mould inoculated castings reported noevidence of subsurface gas holes.

Metallography

All samples were examined for general microstructure. This revealed, inall samples, the standard high chromium hypereutectic white ironmicrostructure of primary M₇ C₃ carbides with a eutectic carbide andferrous matrix, as described already. In the CrC inoculated castingsthere were approximately 0.5 vol % of undissolved CrC particles presentthroughout the casting. A structure similar in appearance to pearlitecolonies was found with varying percentages in each sample. The primaryM₇ C₃ carbide volume in the mould inoculated samples were estimated asranging from 20 to 35%. Total primary carbide volume may be up to 50%.

All samples were also examined for carbide size and the results are setout in Table 5.

The influence on primary carbide size of superheat and amount ofinoculant powder is graphically illustrated for Fe-Cr mould inoculatedsamples in FIG. 6 from which it may be seen that: a) with noinoculation, the primary carbide size varies from about 50 μm with nosuperheat to about 100 μm at 30° C. superheat which agrees well withproduction castings; b) with about 1% inoculant the primary carbide sizeis reduced by about 40 μm at all superheats, a 1° C. increase insuperheat causes 1 μm increase in primary carbide size and 50° C.superheat can still yield a sound casting but with a carbide size ofabout 70 μm; and c) with about 2.5% inoculant very fine primary carbidesizes can be achieved, e.g. about 10 μm at 20° C. superheat, althoughcold folds and gas porosity may present problems at pouring temperaturesof less than about 15° C. superheat, and the influence of the inoculantpowder decreases with increasing contents.

Hardness Results

Vicker's hardness tests were carried out on all samples at 1 mm and 10mm below the cast surface using a 50 kg load. The results aresummarised, along with other results, in Table 5.

From Table 5 is may be seen that there was an average improvement of 67Brinell in the mould inoculated samples A852-A856 and A860 to A865having carbon contents in the range 4.34 to 4.46 wt % at 10 mm below thesurface compared with the standard high chromium hypereutectic whiteiron samples A851 and A866, and a similar increase in hardness at 1 mmdepth. Samples A857 to A859 showed an average increase of 125 Brinell atthe 10 mm depth due to their higher carbon and chromium content. FIG. 7illustrates how the decreasing carbide size increases the grosshardness.

                                      TABLE 5                                     __________________________________________________________________________           Pour  Powder                                                           Sample                                                                            Liqu                                                                             temp                                                                             Sup    Size                                                                             Actual                                                                            Carbide                                                                           Hardness HV50                                                                         Hardness HB                                                                           Vol. %                            no. °C.                                                                       °C.                                                                       °C.                                                                       Type                                                                              mm %   Size μm                                                                        1 mm.sup.1                                                                        10 mm.sup.1                                                                       1 mm.sup.1                                                                        10 mm.sup.1                                                                       Pearlite                          __________________________________________________________________________    A851                                                                              1365                                                                             1373                                                                             8  --  -- 0   70  632 566 594 531 17%                               A852                                                                              "  1375                                                                             10 Fe--Cr                                                                            -75                                                                              1   25  680 672 638 631 App. 10%                          A853                                                                              "  1380                                                                             15 Fe--Cr                                                                            -75                                                                              1.1 25  645 647 606 608                                   A854                                                                              "  1394                                                                             29 Fe--Cr                                                                            -75                                                                              1   45  648 655 609 616                                   A855                                                                              "  1385                                                                             20 Fe--Cr                                                                            -75                                                                              1.1 40  639 635 601 597                                   A856                                                                              "  1381                                                                             16 CrC -150                                                                             1#  40  658 619 618 582                                   A857                                                                              1403                                                                             1428                                                                             25 Fe--Cr                                                                            -75                                                                              1.1 50  713 689 665 646                                   A858                                                                              "  1421                                                                             8  CrC -150                                                                             1.1 45  726 683 673 641                                   A859                                                                              1403                                                                             1429                                                                             26 Fe--Cr                                                                            -75                                                                              1.1 80  777 703 708 657                                   A860                                                                              1364                                                                             1410                                                                             46 Fe--Cr                                                                            -75                                                                              2.5 55  676 635 635 597                                   A861                                                                              "  1398                                                                             34 Fe--Cr                                                                            -75                                                                              2.7 30  629 567 591 532                                   A862                                                                              "  1390                                                                             26 Fe--Cr                                                                            -75                                                                              2.2 30  673 653 632 614                                   A863                                                                              "  1382                                                                             18 Fe--Cr                                                                            -75                                                                              2.3 40  709 677 662 635                                   A864                                                                              "  1390                                                                             26 Fe  -200                                                                             1.2 45  625 601 587 564 27%                               A865                                                                              "  1382                                                                             18 Fe  -200                                                                             1.1 40  631 621 593 584                                   A866                                                                              "  1385                                                                             21 --  -- 0   80  573 562 538 527                                   __________________________________________________________________________     Notes                                                                         .sup.1 Depth below cast surface                                               .sup.2 # Allowing for losses.                                            

Wear Test

Eductor wear tests were carried on ten of the sixteen samples as shownin Table 6 with the tests being performed at a 30° angle and a velocityof 20 m/s. The testing was carried out using 10 kg of medium SilicaRiver Sand (SRS) W300 d85 (485 μm). Wear rate 1 was measured at thesurface of the sample while wear rate 2 was measured in from the castsurface.

As noted previously, samples A851 and A866 are of standard high chromiumhypereutectic white iron with no inoculant while samples A858 and A859are from high carbon and high chromium melts.

FIG. 8 graphically illustrates the trend to improved wear resistancewith finer primary carbides in the SRS W300 wear medium.

In conclusion, it has been shown that all three of powder provedeffective, although there are possible disadvantages with Fe powder dueto the high percentage of pearlite formed. However, these disadvantagesmay be eliminated with a small change in the melt composition or by theuse of heat treatment.

                  TABLE 6                                                         ______________________________________                                               Inoculant   Wear Rate Average  Carbide                                        Powder and  (mm  3/kg)                                                                              Wear Rate                                                                              Size                                    Sample Superheat °C.                                                                      1      2    (mm  3/kg)                                                                             μm                                 ______________________________________                                        A851   none 8      2.10   2.56 2.33     70                                    A852   1%FeCr 10   1.81   1.89 1.85     25                                    A853   1%FeCr 15   1.85   1.99 1.92     25                                    A855   1%FeCr 20   2.56   2.22 2.39     40                                     A858* 1%CrC 16    2.59   1.94 2.27     50                                     A859* 1%FeCr 10   2.62   2.52 2.57     80                                    A861   2%FeCr 34   2.14   2.12 2.13     30                                    A863   2%FeCr 18   2.18   2.00 2.09     40                                    A865   1%Fe 18     2.35   1.91 2.13     40                                    A866   none 21     2.63   2.13 2.38     80                                    ______________________________________                                    

Example 6

FIG. 8 also illustrates the further improvement in wear rate following aheat treatment of four of the samples of Example 5, as shown in Table 7.Eductor wear test conditions were the same as in Example 5. The heattreatment was carried out by heating the castings to 950° C. and holdingfor 4.5 hours, followed by air cooling.

                  TABLE 7                                                         ______________________________________                                               Inoculant  Wear Rate  Average  Carbide                                        Powder and (mm  3/kg) Wear Rate                                                                              Size                                    Sample Superheat °C.                                                                     1      2     (mm  3/kg)                                                                             μm                                 ______________________________________                                        A851   none 8     1.78   2.07  1.93     70                                    A852   1%FeCr 10  1.55   1.61  1.58     25                                     A858* 1%CrC 16   1.86   1.46  1.66     50                                    A865   1%Fe 18    1.81   1.58  1.70     40                                    ______________________________________                                    

As discussed in Example 7, the wear rate increased following heattreatment due to an increase in the hardness of the ferrous matrix. Nocracks were noted in the heat treated samples.

Example 7

This example considered the effect of heat treating three high chromiumhypereutectic white iron castings which have been mould inoculated withabout 1% final casting weight of minus 75 μm Fe-Cr powder and poured atsuperheats of from 25° to 27° C. The after-casting heat treatmentcomprised heating the castings to 950° C. and holding for 45 hours,followed by air cooling.

The castings were of various pump parts and all had the same wt %composition of Cr 30.7, C 4.5, Mn 2, Si 0.57, Mo 0.94, Ni 0.57, B O, S0.03, P 0.04 Fe balance. The melt was the same for all castings and hada liquidus of 1355° C. The castings were tested 1) by visualexamination, 2) by metallograph and 3) for hardness, all both before andafter heat treating.

From the visual examination, all fracture faces showed an appearancetypical of a fine grained structure of a high chromium hypereutecticwhite iron, with no cracks before or after heat treatment.

The microstructures were typical of a high chromium hypereutectic whiteiron with fine primary carbide sizes of 20-25 μm cross-sectionaldimension uniformly spread throughout the matrix. The results of theanalyses and details of the matrices are set out in Tables 8 and 9,respectively.

                                      TABLE 8                                     __________________________________________________________________________                                       Microhardness.sup.1 HB                                                                 Gross                                               Carbide                                                                              Gross Hardness.sup.1                                                                         Matrix                                                                            hardness                                   Pouring                                                                            Supheat                                                                           Size                                                                             Vol Vickers                                                                           Brinell                                                                             Primary                                                                            ferrous                                                                           increase                          Sample                                                                            Condition                                                                          temp °C.                                                                    °C.                                                                        μm                                                                            %.sup.3                                                                           HV50                                                                              Conversion                                                                          Carbide                                                                            phase                                                                             HB                                __________________________________________________________________________    1   As cast                                                                            1380 25  25 25  606.sup.2                                                                         569.sup.2                                                                           1527 536                                   1   Heat "    "   20 25  721.sup.                                                                          671.sup.                                                                            1566 653 102                                   Treated                                                                   2   As cast                                                                            1382 27  25 25  653.sup.                                                                          614.sup.                                                                            1564 442                                   2   Heat "    "   20 25  735.sup.                                                                          681.sup.                                                                            1663 617  67                                   Treated                                                                   3   As cast                                                                            1380 25  25 25  628.sup.                                                                          590.sup.                                                                            1426 405                                   3   Heat "    "   25 25  798.sup.                                                                          720.sup.                                                                            1537 637 130                                   Treated                                                                   __________________________________________________________________________     Notes                                                                         .sup.1 Hardness 10 mm below surface.                                          .sup.2 Hardness 5 mm below surface.                                           .sup.3 Estimated                                                         

                  TABLE 9                                                         ______________________________________                                        Sample                                                                              Condition Matrix                                                        ______________________________________                                        1     As cast   eutectic carbides, martensite and retained                                    austenite with some fine 2° carbide precipitates       1     Heat Treated                                                                            eutectic carbides, martensite and fine 2° carbide                      precipitates                                                  2     As cast   eutectic carbides, martensite and retained                                    austenite                                                     2     Heat Treated                                                                            eutectic carbides, martensite and fine 2° C.                           carbide precipitates                                          3     As cast   eutectic carbides, martensite and retained                                    austenite                                                     3     Heat Treated                                                                            eutectic carbides, martensite and fine 2° C.                           carbide precipitates                                          ______________________________________                                    

The gross hardness results showed that the heat treated samples had anincrease in hardness of from 67 to 102 Brinell, and this is depictedgraphically in FIG. 9. Analyzing the microhardness of the castingsestablished that the increase in gross hardness was due to the increasein hardness of the ferrous matrix. Wear tests in previous examples haveshown that higher hardness achieved by heat treatment increases the wearresistance.

It will be appreciated from the preceding description that a substantialadvantage of the casting method of the present invention as applied tohigh chromium hypereutectic white iron is that a relatively small M₇ C₃primary carbide cross-sectional size can be readily achieved in aninexpensive, quick and uncomplicated manner using existing castingequipment. This is achieved by introducing a particulate material to themolten metal composition at the last possible moment, actually duringthe pour of the melt into the casting mould, to achieve a degree ofundercooling which in turn promotes the formation of the fine grainstructure by maximising the number of primary carbide nuclei and therebyminimising their growth. The addition of the cooling powder in this wayallows a greater pouring window for the casting which is highlybeneficial in foundry practice. It also allows substantially largercastings, for example upto 3000 kg, to be poured than has been possiblein the past without cracking. Past practice has only achieved 100 μmmean cross-sectional primary carbides in 100 mm cross-sectional castingswithout internal chills. Similar sized and larger tough castings can bereadily made by the present invention with a primary carbide meancross-section of 50 μm and less, preferably in the range 20-30 μm.Advantageously these microstructure can be achieved with carbon contentsof 5.5 wt % and higher leading to increased carbide volumes and wearresistance. The relatively small primary carbide size increases the wearresistance of the castings and the fracture toughness, as well asallowing heat treatments to be performed to further increase thehardness and wear resistance. The skilled person in the art willappreciate that many modifications and variations are possible withinthe broad invention, and all such modifications and variations should beconsidered as within the scope of the present invention. In particularit will be appreciated that the invention is applicable to othereutectic alloy systems in which a primary phase grows out of the melt.

We claim:
 1. A method of casting a metal alloy which comprises a primaryphase dispersed in a eutectic phase, the method comprising:(a) forming amelt of the metal alloy; (b) pouring the molten metal alloy at atemperature at least above the liquidus temperature in a stream into acasting mould to form a casting; and (c) introducing a particulatematerial into the stream of molten metal to extract heat from the moltenmetal alloy to undercool the molten metal alloy from the pourtemperature into the primary phase solidification range between theliquidus and the solidus temperatures of the metal alloy to provide, ina the casting mould, a casting including a primary phase dispersed in aeutectic phase.
 2. A casting method according to claim 1 wherein theparticulate material is introduced uniformly to the melt through thepour.
 3. A casting method according to claim 1 wherein the particulatematerial is injected through a nozzle into the stream of molten metal.4. A casting method according to claim 3 wherein the particulatematerial is injected into the stream of molten material in a carrier gascomprising compressed air.
 5. A casting method according to claim 1wherein the particulate material is introduced to the melt at a rate inthe range of 0.1 to 10% of the casting weight.
 6. A casting methodaccording to claim 5 wherein the amount of particulate material is nomore than 5% of the final casting weight.
 7. A casting method accordingto claim 6 wherein the amount of particulate material is in the range0.5 to 1% of the final casting weight.
 8. A casting method according toclaim 1 wherein the maximum particle size of the particulate material is200 μm.
 9. A casting method according to claim 1 wherein the minimumparticle size of the particulate material is 5 μm.
 10. A casting methodaccording to claim 1 wherein the mean particle size of the particulatematerial is in the range 20 to 100 μm.
 11. A casting method according toclaim 1 wherein the particulate material is a powder.
 12. A castingmethod according to claim 1 wherein the particulate material is selectedfrom the group consisting of metals, inorganic metal compounds andalloys.
 13. A casting method according to claim 1 wherein theparticulate material at least partially dissolves in the melt.
 14. Acasting method according to claim 1 wherein the particulate material hasa higher melting point than the melt.
 15. A casting method according toclaim 1 wherein particles of the particulate material are at leastpartly absorbed within the primary phase.
 16. A casting method accordingto claim 1 wherein the particulate material has a compatiblecrystallographic structure with the primary phase.
 17. A casting methodaccording to claim 1 wherein the metal alloy is a high chromiumhypereutectic white iron and the primary phase consists of M₇ C₃carbides.
 18. A casting method according to claim 17 wherein the alloyhas a composition by wt % about 3 to 8.5% C, 20 to 45% Cr, up to 15% Mn,up to 3% Si, up to 10% Mo, up to 10% Ni, up to 5% Cu, up to 2% B, up to1% P, up to 1% S, balance Fe and incidental impurities.
 19. A castingmethod according to claim 18 wherein the alloy has a composition by wt %consisting of 4 to 5.5% C, 28 to 37% Cr, 1 to 4% Mn, 0.1 to 1% Si, 0.5to 1.5% Mo, <1% Ni, <0.1% P, <0.1% S, balance Fe and incidentalimpurities.
 20. A casting method according to claim 17 wherein theprimary M₇ C₃ carbide volume is at least 20% and the primary M₇ C₃carbides are uniformly spread throughout the casting.
 21. A castingmethod according to claim 17 wherein the mean primary M₇ C₃cross-sectional dimension is in the range 10 to 50 μm.
 22. A castingmethod according to claim 21 wherein the dimension is in the range 20 to30 μm.
 23. A casting method according to claim 17 wherein theparticulate material is selected from the group consisting of highcarbon ferrochrome, chromium, carbide and iron.
 24. A casting methodaccording to claim 17 wherein the pour temperature (°C.) isapproximately equal to

    ______________________________________                                        Liquidus (°C.) + A + 15B                                               ______________________________________                                        where A                                                                              =     15° C. for casting section thickness less than 50 mm             =     10° C. for casting section thickness from 50 to 100                    mm                                                                      =     5° C. for casting section thickness greater than 100                   mm.                                                              B      =     amount of particulate material in weight %.                      ______________________________________                                    


25. A casting method according to claim 17 wherein following casting thecasting is subjected to a heat treatment which increases the hardness ofthe matrix.
 26. A casting method according to claim 25 wherein the heattreatment comprises soaking the casting at from 750° to 1050° C. for 2to 5 hours followed by air or furnace cooling.
 27. A casting methodaccording to claim 1 wherein the maximum particle size of theparticulate material is 75 μm.